Add comment to explain why we're using sub_dirs by hand

src/rebar_xref.erl

@@ -133,6 +133,11 @@ check_query({Query, Value}) ->
     end.
 
 code_path(Config) ->
+    %% Slight hack to ensure that sub_dirs get properly included
+    %% in code path for xref -- otherwise one gets a lot of undefined
+    %% functions, even though those functions are present as part
+    %% of compilation. H/t to @dluna. Long term we should tie more
+    %% properly into the overall compile code path if possible.
     BaseDir = rebar_config:get_xconf(Config, base_dir),
     [P || P <- code:get_path() ++
               [filename:join(BaseDir, filename:join(SubDir, "ebin"))